Rate gyroscopes operate on the principle of inertia. Standing waves are excited in a resonating element to produce a desired mode of oscillation having a predetermined number of nodes. The oscillations have an amplitude, a frequency, and an inherent oscillatory inertia that is independent of the linear and rotational inertia of the gyroscope itself. When the resonating element is rotated about its sensing axis, the oscillations will in large part maintain their absolute spatial orientation. However, in maintaining their absolute spatial orientation, the nodes that define the desired mode of oscillation will rotate with respect to the physical structure of the resonating element. This rotation of the nodes is proportional to the physical rotation applied to the resonating element. Taking advantage of this phenomena, it is possible to measure the rate of rotation and determine the magnitude and direction of the rotation that the resonating element has been subjected to.
Solid state gyroscopes based on the principle described above are capable of sensing rotation only, and then only about a single axis. To obtain information sufficient to determine the relative attitude of a body, it is necessary to group at least three such gyroscopes in an orthogonal relationship covering the x, y, and z Cartesian axes.